1. Field of the Invention
The present invention relates to a magneto-optic modulator and an optical communication system using the same.
2. Description of the Related Art
An electro-optic modulator such as a Pockels cell has been widely employed in conventional optical communication systems. Particularly, a wave-guide optical modulator utilizing an electro-optic effect of the LiNbO3 crystal is a typical one (Nishihara et al., Optical Integrated Circuit, pp298-304, 1985, Ohm-sha). However, the optical modulator using the electro-optic crystal has a disadvantage that it suffers from the DC drift (J. Appl. Phys. Vol. 76 No. 3 pp1405-1408 (1994)) and optical damage. Therefore, it is difficult to operate it stably for a long time period, or it costs much to avoid a deterioration in its characteristics.
Recently, there have been disclosed various optical communication systems wherein an electric field from an antenna as a source of radio frequency signal is applied to the electro-optic modulator (JP4-1722611A, JP10-186189 A).
On the other hand, although a magneto-optic modulator has been studied for a long time, development thereof is not being well advanced, due to its response speed slower than that of the electro-optic modulator.
Therefore, the magneto-optic modulator is applied, due to the slow response speed, only to a magnetic field sensor or electric current sensor which operates at a low response speed (J. Appl. Pyhs. Vol. 53 No.11 pp.8263-8265 (1982), National Technical Report Vol. 38 No. 2, pp. 127-133 (1992)).
The conventional magneto-optic modulator employed in an optical communication system disclosed in JP7-199137A responds to no higher than several tens kHz. Further, an optical isolator is used for a magneto-optic modulator in U.S. Pat. No. 6,141,140. However, the response speed thereof is also low and one of the disadvantages thereof is that it does not operate as the optical isolator, due to a light unnecessarily reflected-back toward a light source under the presence of Faraday effect.
Further, a magneto-optic modulator wherein a DC bias magnetic field is applied to a magneto-optic crystal film has been recently studied in order to measure an electric current in a semiconductor electronic circuit substrate (Appl. Phys. Lett. Vol. 68, No. 25 pp. 3546-3548 (1996), 61th JJAP Transaction, lecture No. 4p-Q-4 (2000)).
As described above, the electro-optic modulator, particularly the wave-guide optical modulator using the Pockels effect has advantages that it is suitable for a high speed modulation of a laser or LED light and that it is free from a wave-length variation or wave-length chirping which is caused by a direct modulation of a semiconductor laser. However, the electro-optic modulator has a disadvantage that it has a DC drift and optical damage which increase a production cost in order to countermeasure against the disadvantage. Further, the DC drift and temperature characteristics are of great concerns in the optical communication system wherein the light is modulated by the electric signal from the antenna in the optical modulator disposed outdoors.
Further, there is a magneto-optic modulator for monitoring an electric current wave form on a micro strip line, by disposing the magneto-optic crystal directly on a semiconductor substrate or micro strip line and by applying a DC bias magnetic field to the magneto-optic crystal (Appl. Phys. Lett. Vol. 68 No. 25 pp. 3546-3548 1996). However, the above-mentioned current monitoring has a disadvantage that the current wave form is distorted by a ringing due to an impedance mismatching between the line and modulation signal generator. Further, the above-mentioned current monitoring device does not include any optical fiber and therefore, is not suitable for the optical communication systems. On the other hand, another magneto-optic modulator for monitoring the current wave form on the micro strip line wherein an analyzer is disposed after passing a short, e.g., shorter than about 1 m, optical fiber (61st JJAP Transaction, Lecture No. 4p-Q-4 (2000)). However, a linear polarization becomes in general a random polarization through a long optical fiber. Therefore, the intensity modulation of light propagating through a long optical fiber can not be achieved even by using the analyzer. Further, in the above-mentioned another magneto-optic modulator, the DC bias magnetic field is almost parallel to the radio frequency magnetic field. Therefore, the above-mentioned another magneto-optic modulator has a disadvantage that the magneto-optic modulator is magnetically saturated under a large bias magnetic field for obtaining a single magnetic domain and the magnetic saturation greatly reduces or completely extinguishes the modulated signal.
It is true that there are being used in almost all of the conventional optical communication system a direct high speed modulation of the electric current in the semiconductor laser and a wave-guide optical modulator utilizing the electro-optic effect (Pockels effect). Although the direct modulation of the semiconductor laser has an advantage that the optical communication system does not need any modulator and therefore, its structure becomes simple, the modulation frequency is no higher than several GHz and driving circuit becomes highly advanced and a transmission distance through the optical fiber of the optical signal is limited by the wavelength chirping due to the high speed direct modulation which causes group velocity delay differences depending the chirped wavelengths, thereby distorting a signal propagating the long optical fiber.